The present invention relates to a system for controlling the air-fuel ratio for an internal combustion engine emission control system having a three-way catalyst, and more particularly to a system which effectively controls the air-fuel ratio at acceleration of an engine for a vehicle.
Such a system is a feedback control system, in which an O.sub.2 sensor is provided to sense the oxygen content of the exhaust gases to generate an electrical signal as an indication of the air-fuel ratio of an air-fuel mixture supplied by a carburetor. The control system comprises a comparator for comparing the output signal of the O.sub.2 sensor with a predetermined value, a proportional and integrating circuit connected to the comparator, a driving circuit for producing square wave pulses from the output signal of the proportional and integrating circuit, and an on-off type electromagnetic valve for correcting the air-fuel ratio of the mixture. The comparator operates to judge whether the feedback signal from the O.sub.2 sensor is higher or lower than a predetermined reference value corresponding to the stoichiometric air-fuel ratio for producing an error signal and the signal is integrated by the proportional and integrating circuit to produce an integrated output. The integrated output is converted to pulses for actuating the on-off electromagnetic valve to thereby control the air-fuel ratio of the mixture.
In such a control system, the feedback control is not effected during rapid acceleration with a wide-open throttle (WOT) engine operation and a pulse train having a predetermined duty ratio is produced for supplying a rich air-fuel mixture for the purpose of performance of the rapid acceleration. When the acceleration finishes and the throttle valve returns to a part open throttle condition, the feedback control operation becomes effective again. At the moment of re-start of the feedback control operation, the integrating circuit starts to integrate the input signal from the minimum value set in the circuit. Consequently, a longer time than the normal control condition is elapses until the integrated value reaches a value sufficient to reduce the rich air-fuel mixture during the acceleration to a lean air-fuel mixture for controlling the air-fuel ratio. As a result, controlling the air-fuel ratio to the stoichiometric air-fuel ratio is delayed.